Micro-miniature, solid state accelerometers are used for a number of important applications, such as for acceleration sensors in missile safe and arm devices. One prior solid state accelerometer comprises a mass supported by a silicon beam upon which one or more piezoresistive sensing elements are formed. Under acceleration, the restoring force exerted by the beam on the mass induces stress in the sensing element. The resistance of the sensing element changes with the stress, and the change in resistance is converted to a differential voltage by using one or two sensing elements in a resistance bridge circuit.
The main problem with accelerometers using silicon piezoresistive sensing elements is temperature sensitivity. Doped silicon has a temperature coefficient of resistance of about several thousand parts per million per degree centigrade. The sensing elements therefore experience a change in resistance due to temperature changes as well as due to stress caused by acceleration. The temperature sensitivity of the output voltage can be reduced by completing the resistance bridge with resistors formed on the silicon beam adjacent to the sensing elements, but aligned such that they are insensitive to the stress. Although this arrangement works reasonably well, it nevertheless does not eliminate problems with temperature gradients across the silicon. In addition, the current through the bridge varies significantly over temperature, and trimming resistors located off the bridge must have temperature characteristics that track those of silicon.
Another prior solid state accelerometer design comprises a cantilevered beam of silicon dioxide fabricated on the surface of a silicon wafer and suspended over a well etched in the surface. A mass of metal is deposited on one side of the beam to provide sufficient sensitivity to acceleration. However, in this design the center of mass is offset from the centerline plane of the beam, thereby creating a sensitivity to accelerations in directions other than the intended sensitive direction.
Another prior solid state accelerometer design comprises a semiconductor flap member fixed to one side of a torsion bar. An electrode is located on a separate substrate that is attached to the flap member after fabrication. Acceleration is measured by comparing the variable capacitance between the electrode and the flap member with a fixed capacitance. This design has a number of drawbacks. Semiconductor materials such as silicon and silicon dioxide have relatively low densities. An inertial element made from silicon or silicon dioxide therefore has less mass and is less sensitive to acceleration than one of equal size made from heavier materials such as metals. Further drawbacks are that the voltage that must be applied to the variable capacitor to measure the capacitance creates an electrostatic force on the movable flap member that disturbs the value being measured. A further undesirable feature of this design is that the accelerometer requires the fabrication and assembly of two separate substrates. In addition, comparison of a variable with a fixed capacitance can introduce temperature sensitivities if the thermal coefficients of the two capacitors are not carefully matched. Finally, attachment of a torsion bar at two different locations onto a substrate of a material with a different thermal coefficient of expansion can introduce changes in longitudinal stress in the torsion bar due to changes in temperature, and such stress changes can result in a temperature sensitive scale factor for the accelerometer.